Which organs are part of the endocrine system?

• A. Lymph nodes, thymus, spleen, tonsils
• B. Kidneys and bladder
• C. Heart and blood vessels
• D. Pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, gonads

Answer: (D)

Endocrine signaling relies on glands that release hormones directly into the bloodstream to regulate processes like growth, metabolism, and reproduction. The pituitary, thyroid, parathyroid, adrenal glands, pancreas, and gonads are classic endocrine glands—they produce hormones such as growth hormone, thyroid hormones, parathyroid hormone, cortisol, insulin, glucagon, and sex hormones that travel through the blood to affect distant organs. The other organs listed mainly have nonendocrine roles: lymph nodes, thymus, spleen, and tonsils are part of the immune system; kidneys and bladder are part of the urinary system (kidneys do have some hormonal outputs, but their primary function is filtration); the heart and blood vessels belong to the circulatory system (the heart can release a hormone in some contexts, but it’s not treated as a primary endocrine gland). Because the primary function of the glands named is hormone production that coordinates body-wide processes, they constitute the endocrine system.